For a thorough understanding of the present subject a very extensive knowledge of mathematics is necessary, but there are parts of the subject in which a reader with only a limited mathematical equipment may soon feel at home and perhaps do useful original work. With the idea of enabling such a reader to obtain a quick grasp of the nature of the subject and the results obtained, I have thought it advisable to state without proof a number of relations of which adequate demonstrations can only be obtained by means of complicated and difficult analysis. I have also endeavoured to keep the analysis as elementary as possible, but in some places where the work is perfectly straight forward a few details are omitted.